In March 1986, Voyne Ray Cox was a patient in the East Texas Cancer Center for Tyler, Texas. Ray, as he was commonly called, had surgery to remove a tumor from his left shoulder and was receiving follow-up radiation treatments. He had every reason to believe that between the surgery and the radiation treatments he would overcome his cancer and return to his work in the oil fields. His first eight radiation treatments had gone well, and he did not believe that the ninth session would cause him any problems.
The cancer ceenter had state-of-the-art radiation equipment. The Therac-25 was a million-dollar piece of equipment capable of delivering a beam of high-energy radiation to any point on or in the human body. The key to successful radiation treatment was twoifold: hitting the cancer cells with pinpoint accuracy and administering many separate treatments involving relatively low doses rather than a single treatment with one large dose. ray was receiving the small doses; his schedule called for many more sessions in the tretment room.
The technician was very familiar with the Digital Equipment Corporation VT100 terminal connected to a PDP-11 computer to control the radiation. Using the control system, the technician could aim the accelerator with pinpoint accuracy and fire a direct readiation beam off the presecirbed intensisty at the site. Ray should not have felt a thing........ However, the video camera did not function in the technician control room on the day Ray's ninth radiatio"n treatment because it was not plugged in. The voice intercom between the two rooms was also inoperative. But neither the technician nor Ray seemed concerned about these communication devices because all previous treatments had taken place without incident.
According to Casey (1993) the Therac-25 had two modes of operation. A high-powered x-ray mode utilized the full 25 million electron volt capacity of the machine. "It was selected by typing an 'x' on the keyboard. This put the machine on maximum power and automatically inserted a thick metal plate just beneath the beam.When passed through the metal plate, the beam transformed into an x-ray, which irradiated tumors inside the Body. The plate also lowered the intensity of the beam." The other setting lowered the power mode selected by pressing 'e' on the keyboard. Ray was scheduled to receive the second option: a painless, low dose of about 200 rads to the spot on his shoulder. After the technician carefully positioned Ray on the metal treatment table, she entered the control room to begin the treatment. She typed in 'x' and immediately relaized that she had entered the wrong mode. She had intended to enter an 'e' for the lower dose setting but had entered 'x', the higher x-ray setting. Because the treatment had not begun, the technician corrrected herr error by following the guidelines established by Therac's manufacturere. She used the 'up' key to select the edit functions and corrected the electron beam setting. The screen display verified that she now had the correct setting. She had corrected her initial error within 8 seconds.
Unfortunately, the designers of the Therac-25 Atomic Energy of Canada, Ltd. (AECL), had never considered the possibility of that sequence of key strokes occurring in that short a segment of time. In the thousands of treatments already delivered by the Therac-25 machine, this sequence of inputs had never occurred; therefore, it had never been tested. Instead of responding as expected and correcting the input, error, the machine retracted the metal barrier plate for the x-ray mode but proceeded to deliver a full 25,000 rads to Ray's shoulder when the technician pressed the beam key. This was equivalent to 25 million electron volts.
Ray saw a flash of blue light, heard a frying sound, and felt an excruciating pain like ahot poker in his back. He rolled to his side, feeling that his shoulder was on fire. Inside the control room, the technician wa unaware of Ray's plight because the communictaion with the treatment room was not functioning. Her computer screen did light up with a malfunction message indicating that the machine was not functioning and that treatment had NOT been initiated. The techniician quicly reset the machine and fired it again. The second flash of lue light and the radiation blast caught Ray in the neck. the pain was more intense than anything he had ever endured. He tried to call out, but the technician could not hear him. The machine still dsiplayed an error message. A third beam of radiation hit Ray before he rolled off the table and fled the room.
On examination, no physical injury of Ray was apparent, and no malfunction was apparent in the machine. The hospital continued to use the machine that day because other patients were waiting for their treatments. Three weeks later, the same sequence of events produced a blue ight and extreme pain in another patient. At this time the AECL and other users of the Therac-25 were alerted to the problem. Subsequent investigation found similar overdoses in clinics in Marietta, Georgia; Ontario, Canada; and Yakima, Washington. Four months later, Ray died of massive radiation poisoning. [1]
Victim #1: Linda Knight
In June 1985 61-year old Linda Knight had been receiving follow-up treatment at the Kennestone Regional Oncology Center (Marietta, GA) for the removal of a malignant breast tumor. On June 3, staff at Kennestone prepared Knight for electron treatment to the clavicle area, using the Therac-25 machine.
Knight had been through the process before, which was ordinarily uneventful. This time, when the machine was turned on, Knight felt a "tremendous force of heat… this red-hot sensation." When the technician re-entered the therapy room, Knight said, "you burned me." The technician replied that that was "not possible."
Back home, the skin above Knight's left breast began swelling. The pain was so great that she checked in at Atlanta's West Paces Ferry Hospital a few days after the Therac incident. For a week, doctors at West Paces Ferry continued to send Knight back to Kennestone for Therac treatment, but when the welt on her chest began to break down and lose layers of skin, Knight refused to undergo any more radiation treatment.
About two weeks later, the physicist at Kennestone noticed that Knight had a matching burn on her back, as though the burn had gone through her body. The swelling on her back had also begun to slough off skin. Knight was in great pain, and her shoulder had become immobile. These clues led the physicist to conclude that Knight had indeed suffered a major radiation burn. Knight had probably received one or two radiation doses in the 20,000-rad (radiation absorbed dose) range, well above the typical prescribed dosage of around 200-rads. The physicist called AECL and, without telling of the accident, asked questions about the likelihood of radiation overexposure from the Therac 25 machine: Could Therac 25 operate in electron mode without scanning to spread the beam? Three days later AECL engineers called back to say this was not possible.
Linda Knight was in constant pain, lost the use of her shoulder and arm, and her left breast had to be removed because of the radiation burns.
Victim #2: Donna Gartner
In July, 1985 Donna Gartner, a 40-year old cancer patient, was at the Ontario Cancer Foundation clinic in Hamilton, Ontario, Canada for her 24th Therac treatment for carcinoma of the cervix.
The Therac-25 operator activated the machine, but after 5 seconds, the Therac-25 shut down and showed an "H-tilt" error message. The computer screen indicated that no dose had been given, so the operator hit the "P" key for the "proceed" command. The Therac shut down in the same manner as before, reading "no dose," so the operator repeated the process a total of four times after the initial try.
After the fifth try, a hospital service technician was called but found no problems with the machine. Donna Gartner left the clinic and the Therac was used with six other patients that day without any incidents. However, despite the fact that the Therac had indicated that no radiation dose had been given during Donna Gartner's five therapy attempts that day, Gartner complained of a burning sensation she described as an "electric tingling shock" in the treated area of her hip.
Gartner returned for treatment three days later, on July 29, and was hospitalized for suspected radiation overexposure. She had considerable burning, pain and swelling in the treatment region of her hip. The Hamilton clinic took the Therac-25 machine out of service and informed AECL of the incident. This was the first time AECL had heard from a clinic about an overdose problem with the Therac-25 machine. AECL sent a service engineer to investigate.
AECL reported to a range of stakeholders that there was a problem with the operation of Therac 25. The FDA, the Canadian Radiation Protection Board (the parallel Canadian agency to the FDA), and other Therac-25 users were all notified. Users were instructed to visually confirm that the Therac turntable was in the correct position for each use.
Because of the Hamilton accident, AECL issued a voluntary recall of the Therac-25 machines and the FDA audited AECL's modifications to the Therac. AECL could not reproduce the malfunction that had occurred but suspected some hardware errors in a switch that monitored the turntable position. A failure of this switch could result in the turntable being incorrectly positioned, and an unmodified electron beam striking the patient. The company redesigned the mechanism used to lock the turntable into place, redesigned the switch to detect position and it accompanying software. They then reported in November 1985 that this redesign was complete and that, given their safety analyses, the machine was now at least 10,000 times safer than before.
Donna Gartner died on November 3, 1985 from cancer. An autopsy revealed that had the cancer not killed Gartner, a total hip replacement would have been necessary because of the radiation overexposure.
Victim #3: Janis Tilman
Janis Tilman was being treated with the Therac-25 machine at the Yakima Valley Memorial Hospital in Yakima, Washington. After one treatment in December 1985, her skin in the treatment area, her right hip, began to redden in a parallel striped pattern. The reddening did not immediately follow treatment with the Therac-25 because it generally takes at least several days before the skin reddens and/or swells from a radiation overexposure.
Tilman continued Therac treatment until January 6, 1986 despite the reddening, since it was not determined that the reddening was an abnormal reaction. Hospital staff monitored the skin reaction and searched unsuccessfully for possible causes for the striped marks.
The hospital sent a letter to AECL and spoke on the phone with AECL's technical support supervisor, who later sent a written response stating, "After careful consideration, we are of the opinion that this damage could not have been produced by any malfunction of the Therac-25 or by any operator error." The hospital staff dismissed the skin/tissue problem as "cause unknown," partly due to the response from AECL, and partly because they knew AECL had already installed additional safety devices to their Therac-25 machine in September 1985.
Upon investigation in February 1987, the Yakima staff found Tilman to have a chronic skin ulcer, dead tissue, and constant pain in her hip, providing further evidence for a radiation overexposure. Tilman underwent surgery and skin grafts, and overcame the incident with minor disability and some scarring related to the overdose.
Victim #4: Isaac Dahl
March 22, 1986. At the East Texas Cancer Center (ETCC) in Tyler, Texas, 33-year old Isaac Dahl was to receive his ninth Therac-25 radiation therapy session after a tumor had been successfully removed from his left shoulder. By this time the Therac 25 had been in successful operation at Tyler for two years, and 500 patients had been treated with it.
The Therac-25 operator left the radiation room to begin the treatment as usual. As she was typing in values, she made a mistake and used the "cursor up" key to correct it. Once the values were correct, she hit the "B" key to begin treatment, but the Therac-25 machine shut down after a moment, and the message "Malfunction 54" showed on the control room monitor. The machine indicated that only 6 of the prescribed 202 units of radiation had been delivered. The screen of the console showed that this shut down was a "treatment pause" which indicated a problem of low priority (since little radiation had been delivered). The operator hit the "P" key to proceed with the therapy, but after a moment of activity, "Malfunction 54" appeared on the Therac control screen again.
The operator was isolated from Dahl because the Therac-25 operates from within a shielded room. On this day at the ETCC, the video monitor was unplugged and the audio monitor was broken, leaving no way for the operator to know what was happening inside. Isaac Dahl had been lying on the treatment table, waiting for the usually uneventful radiation therapy, when he saw a bright flash of light, heard a frying, buzzing sound, and felt a thump and heat like an electric shock.
Dahl, knowing from his previous 8 sessions that this was not normal, began to get up from the treatment table when the second "attempt" at treatment occurred. This time the electric-like jolt hit him in the neck and shoulder. He rolled off the table and pounded on the treatment room door until the surprised Therac-25 operator opened it. Dahl was immediately examined by a physician, who observed reddening of the skin but suspected only an electric shock. Dahl was discharged and told to return if he suffered any further complications.
The hospital physicist was called in to examine the Therac-25, but no problems were found. The Therac-25 was shut down for testing the next day, and two AECL engineers, one from Texas and one from the home office in Canada, spent a day at the ETCC running tests on the machine but could not reproduce a Malfunction 54. The home office engineer explained that the Therac-25 was unable to overdose a patient and also said that AECL had no knowledge of any overexposure accidents by Therac-25 machines. No electrical problems were found with the ETCC's Therac machine, and it was put back into use on April 7, 1986.
Isaac Dahl's condition worsened as he lost the use of his left arm and had constant pain and periodic nausea and vomiting spells. He was later hospitalized for several major radiation-induced symptoms (including vocal cord paralysis, paralysis of his left arm and both legs, and a lesion on his left lung). Dahl died in August of 1986 due to complications from the radiation overdose.
Victim #4: Daniel McCarthy
April 11, 1986. Technicians could find nothing wrong with the Therac-25 unit at the East Texas Cancer Center (ETCC), after the "Malfunction 54" incident that had injured Isaac Dahl. The machine was reinstated.
Four days later, Daniel McCarthy was being treated for skin cancer on the side of his face. The same Therac operator who had treated Isaac Dahl was treating McCarthy. As the operator prepared to administer the Therac treatment from the control room, she used the "cursor up" key to correct an error in the treatment settings. She then began treatment using the "B" key.
The Therac-25 shut down within a few seconds, making a noise audible through the newly repaired intercom. The Therac monitor read "Malfunction 54." The operator rushed into the treatment room and found McCarthy moaning for help. He said that his face was on fire. The hospital physicist was called. McCarthy said that something had hit the side of his face, and that he had seen a flash of light and heard a sizzling sound.
After this second accident at the hospital, the ETCC physicist took the Therac-25 out of service and called AECL. He worked with the Therac operator who had been administering treatment to both Dahl and McCarthy when the accidents occurred. The physicist and the operator were eventually able to reproduce a Malfunction 54. They found that the malfunction occurred only if the Therac-25 operator rapidly corrected a mistake.
The ETCC physicist notified AECL of this discovery and AECL was eventually able to reproduce the error. AECL advised Therac-25 users to physically remove the up-arrow key as a short-term solution. AECL also filed a report with the United States FDA as required by law, and began work on fixing the software bug.
The FDA worked in conjunction with AECL to identify the software problem and correct it. The FDA also requested that AECL change the machine in several other ways to clarify the meaning of malfunctions error messages and to shut down treatment after any single large radiation pulse or interrupted treatment so that multiple overdoses were less likely.
Over the next three weeks Daniel McCarthy became very disoriented and then fell into a coma. He had a fever as high as 104 degrees and had suffered neurological damage. He died on May 1, 1986.
Victim #6: Anders Engman
January, 1987, Anders Engman was at the Yakima Valley Memorial Hospital on January 17, 1987 to receive three sets of radiation treatment from the Therac-25.
The first two treatments went as planned. Engman received 7 rads (radiation absorbed dose), 4 rads followed by 3 rads of radiation to take pictures of internal structure. The Therac-25 operator then entered the room and used the Therac-25's hand control to verify proper beam alignment on Engman's body. Engman's final dose of the day was to be a moderate 79-rad photon treatment.
The operator pressed a button to command the Therac to move its turntable to the proper position for treatment. Outside the treatment room, the Therac-25's control console read "beam ready," and the operator pressed the "B" key to turn the beam on. The beam activated, but the Therac-25 shut down after about 5 seconds. The console indicated that no dose had been given, so the operator pressed "P" to proceed with the treatment.
The Therac-25 shut down again, listing "flatness" as the reason for treatment pause. Engman said something over the intercom, but the operator couldn't understand him. The operator went into the treatment room to speak with Engman. Engman told the operator that he had felt a "burning sensation" in the chest. The operator's console displayed only the total dose of the two earlier treatments (7 rads).
Later that day, Engman developed a skin burn over the treatment area. Four days later the burn was striped in a manner similar to that of Janis Tilman's burn after she had been treated at Yakima the year before.
AECL investigated the accident. All users were again told to visually confirm turntable setting before proceeding with any treatment. Given the information, it was suspected that the electron beam had come on when the turntable was in the field light position. AECL could not reproduce the error.
Later that week, AECL sent an engineer to Yakima to investigate. The hospital physicist had also been running tests. They eventually discovered a software flaw and fixed it. AECL engineers estimated that Engman received between 8,000 and 10,000 rads instead of the prescribed 86.
Anders Engman died in April 1987. He had been suffering from a terminal form of cancer before the Therac accident, but it was determined that his death was primarily caused by complications related to the radiation overdose, not the cancer.
^ Casey, S. (1993). Set phasers on stun and other true tales of design, technology, and human error. Santa Barbara, CA: Aegean.
Table of Contents
Victim #5: Ray Voyne Cox
In March 1986, Voyne Ray Cox was a patient in the East Texas Cancer Center for Tyler, Texas. Ray, as he was commonly called, had surgery to remove a tumor from his left shoulder and was receiving follow-up radiation treatments. He had every reason to believe that between the surgery and the radiation treatments he would overcome his cancer and return to his work in the oil fields. His first eight radiation treatments had gone well, and he did not believe that the ninth session would cause him any problems.The cancer ceenter had state-of-the-art radiation equipment. The Therac-25 was a million-dollar piece of equipment capable of delivering a beam of high-energy radiation to any point on or in the human body. The key to successful radiation treatment was twoifold: hitting the cancer cells with pinpoint accuracy and administering many separate treatments involving relatively low doses rather than a single treatment with one large dose. ray was receiving the small doses; his schedule called for many more sessions in the tretment room.
The technician was very familiar with the Digital Equipment Corporation VT100 terminal connected to a PDP-11 computer to control the radiation. Using the control system, the technician could aim the accelerator with pinpoint accuracy and fire a direct readiation beam off the presecirbed intensisty at the site. Ray should not have felt a thing........ However, the video camera did not function in the technician control room on the day Ray's ninth radiatio"n treatment because it was not plugged in. The voice intercom between the two rooms was also inoperative. But neither the technician nor Ray seemed concerned about these communication devices because all previous treatments had taken place without incident.
According to Casey (1993) the Therac-25 had two modes of operation. A high-powered x-ray mode utilized the full 25 million electron volt capacity of the machine. "It was selected by typing an 'x' on the keyboard. This put the machine on maximum power and automatically inserted a thick metal plate just beneath the beam.When passed through the metal plate, the beam transformed into an x-ray, which irradiated tumors inside the Body. The plate also lowered the intensity of the beam." The other setting lowered the power mode selected by pressing 'e' on the keyboard. Ray was scheduled to receive the second option: a painless, low dose of about 200 rads to the spot on his shoulder. After the technician carefully positioned Ray on the metal treatment table, she entered the control room to begin the treatment. She typed in 'x' and immediately relaized that she had entered the wrong mode. She had intended to enter an 'e' for the lower dose setting but had entered 'x', the higher x-ray setting. Because the treatment had not begun, the technician corrrected herr error by following the guidelines established by Therac's manufacturere. She used the 'up' key to select the edit functions and corrected the electron beam setting. The screen display verified that she now had the correct setting. She had corrected her initial error within 8 seconds.
Unfortunately, the designers of the Therac-25 Atomic Energy of Canada, Ltd. (AECL), had never considered the possibility of that sequence of key strokes occurring in that short a segment of time. In the thousands of treatments already delivered by the Therac-25 machine, this sequence of inputs had never occurred; therefore, it had never been tested. Instead of responding as expected and correcting the input, error, the machine retracted the metal barrier plate for the x-ray mode but proceeded to deliver a full 25,000 rads to Ray's shoulder when the technician pressed the beam key. This was equivalent to 25 million electron volts.
Ray saw a flash of blue light, heard a frying sound, and felt an excruciating pain like ahot poker in his back. He rolled to his side, feeling that his shoulder was on fire. Inside the control room, the technician wa unaware of Ray's plight because the communictaion with the treatment room was not functioning. Her computer screen did light up with a malfunction message indicating that the machine was not functioning and that treatment had NOT been initiated. The techniician quicly reset the machine and fired it again. The second flash of lue light and the radiation blast caught Ray in the neck. the pain was more intense than anything he had ever endured. He tried to call out, but the technician could not hear him. The machine still dsiplayed an error message. A third beam of radiation hit Ray before he rolled off the table and fled the room.
On examination, no physical injury of Ray was apparent, and no malfunction was apparent in the machine. The hospital continued to use the machine that day because other patients were waiting for their treatments. Three weeks later, the same sequence of events produced a blue ight and extreme pain in another patient. At this time the AECL and other users of the Therac-25 were alerted to the problem. Subsequent investigation found similar overdoses in clinics in Marietta, Georgia; Ontario, Canada; and Yakima, Washington. Four months later, Ray died of massive radiation poisoning. [1]
Victim #1: Linda Knight
In June 1985 61-year old Linda Knight had been receiving follow-up treatment at the Kennestone Regional Oncology Center (Marietta, GA) for the removal of a malignant breast tumor. On June 3, staff at Kennestone prepared Knight for electron treatment to the clavicle area, using the Therac-25 machine.Knight had been through the process before, which was ordinarily uneventful. This time, when the machine was turned on, Knight felt a "tremendous force of heat… this red-hot sensation." When the technician re-entered the therapy room, Knight said, "you burned me." The technician replied that that was "not possible."
Back home, the skin above Knight's left breast began swelling. The pain was so great that she checked in at Atlanta's West Paces Ferry Hospital a few days after the Therac incident. For a week, doctors at West Paces Ferry continued to send Knight back to Kennestone for Therac treatment, but when the welt on her chest began to break down and lose layers of skin, Knight refused to undergo any more radiation treatment.
About two weeks later, the physicist at Kennestone noticed that Knight had a matching burn on her back, as though the burn had gone through her body. The swelling on her back had also begun to slough off skin. Knight was in great pain, and her shoulder had become immobile. These clues led the physicist to conclude that Knight had indeed suffered a major radiation burn. Knight had probably received one or two radiation doses in the 20,000-rad (radiation absorbed dose) range, well above the typical prescribed dosage of around 200-rads. The physicist called AECL and, without telling of the accident, asked questions about the likelihood of radiation overexposure from the Therac 25 machine: Could Therac 25 operate in electron mode without scanning to spread the beam? Three days later AECL engineers called back to say this was not possible.
Linda Knight was in constant pain, lost the use of her shoulder and arm, and her left breast had to be removed because of the radiation burns.
Victim #2: Donna Gartner
In July, 1985 Donna Gartner, a 40-year old cancer patient, was at the Ontario Cancer Foundation clinic in Hamilton, Ontario, Canada for her 24th Therac treatment for carcinoma of the cervix.The Therac-25 operator activated the machine, but after 5 seconds, the Therac-25 shut down and showed an "H-tilt" error message. The computer screen indicated that no dose had been given, so the operator hit the "P" key for the "proceed" command. The Therac shut down in the same manner as before, reading "no dose," so the operator repeated the process a total of four times after the initial try.
After the fifth try, a hospital service technician was called but found no problems with the machine. Donna Gartner left the clinic and the Therac was used with six other patients that day without any incidents. However, despite the fact that the Therac had indicated that no radiation dose had been given during Donna Gartner's five therapy attempts that day, Gartner complained of a burning sensation she described as an "electric tingling shock" in the treated area of her hip.
Gartner returned for treatment three days later, on July 29, and was hospitalized for suspected radiation overexposure. She had considerable burning, pain and swelling in the treatment region of her hip. The Hamilton clinic took the Therac-25 machine out of service and informed AECL of the incident. This was the first time AECL had heard from a clinic about an overdose problem with the Therac-25 machine. AECL sent a service engineer to investigate.
AECL reported to a range of stakeholders that there was a problem with the operation of Therac 25. The FDA, the Canadian Radiation Protection Board (the parallel Canadian agency to the FDA), and other Therac-25 users were all notified. Users were instructed to visually confirm that the Therac turntable was in the correct position for each use.
Because of the Hamilton accident, AECL issued a voluntary recall of the Therac-25 machines and the FDA audited AECL's modifications to the Therac. AECL could not reproduce the malfunction that had occurred but suspected some hardware errors in a switch that monitored the turntable position. A failure of this switch could result in the turntable being incorrectly positioned, and an unmodified electron beam striking the patient. The company redesigned the mechanism used to lock the turntable into place, redesigned the switch to detect position and it accompanying software. They then reported in November 1985 that this redesign was complete and that, given their safety analyses, the machine was now at least 10,000 times safer than before.
Donna Gartner died on November 3, 1985 from cancer. An autopsy revealed that had the cancer not killed Gartner, a total hip replacement would have been necessary because of the radiation overexposure.
Victim #3: Janis Tilman
Janis Tilman was being treated with the Therac-25 machine at the Yakima Valley Memorial Hospital in Yakima, Washington. After one treatment in December 1985, her skin in the treatment area, her right hip, began to redden in a parallel striped pattern. The reddening did not immediately follow treatment with the Therac-25 because it generally takes at least several days before the skin reddens and/or swells from a radiation overexposure.Tilman continued Therac treatment until January 6, 1986 despite the reddening, since it was not determined that the reddening was an abnormal reaction. Hospital staff monitored the skin reaction and searched unsuccessfully for possible causes for the striped marks.
The hospital sent a letter to AECL and spoke on the phone with AECL's technical support supervisor, who later sent a written response stating, "After careful consideration, we are of the opinion that this damage could not have been produced by any malfunction of the Therac-25 or by any operator error." The hospital staff dismissed the skin/tissue problem as "cause unknown," partly due to the response from AECL, and partly because they knew AECL had already installed additional safety devices to their Therac-25 machine in September 1985.
Upon investigation in February 1987, the Yakima staff found Tilman to have a chronic skin ulcer, dead tissue, and constant pain in her hip, providing further evidence for a radiation overexposure. Tilman underwent surgery and skin grafts, and overcame the incident with minor disability and some scarring related to the overdose.
Victim #4: Isaac Dahl
March 22, 1986. At the East Texas Cancer Center (ETCC) in Tyler, Texas, 33-year old Isaac Dahl was to receive his ninth Therac-25 radiation therapy session after a tumor had been successfully removed from his left shoulder. By this time the Therac 25 had been in successful operation at Tyler for two years, and 500 patients had been treated with it.The Therac-25 operator left the radiation room to begin the treatment as usual. As she was typing in values, she made a mistake and used the "cursor up" key to correct it. Once the values were correct, she hit the "B" key to begin treatment, but the Therac-25 machine shut down after a moment, and the message "Malfunction 54" showed on the control room monitor. The machine indicated that only 6 of the prescribed 202 units of radiation had been delivered. The screen of the console showed that this shut down was a "treatment pause" which indicated a problem of low priority (since little radiation had been delivered). The operator hit the "P" key to proceed with the therapy, but after a moment of activity, "Malfunction 54" appeared on the Therac control screen again.
The operator was isolated from Dahl because the Therac-25 operates from within a shielded room. On this day at the ETCC, the video monitor was unplugged and the audio monitor was broken, leaving no way for the operator to know what was happening inside. Isaac Dahl had been lying on the treatment table, waiting for the usually uneventful radiation therapy, when he saw a bright flash of light, heard a frying, buzzing sound, and felt a thump and heat like an electric shock.
Dahl, knowing from his previous 8 sessions that this was not normal, began to get up from the treatment table when the second "attempt" at treatment occurred. This time the electric-like jolt hit him in the neck and shoulder. He rolled off the table and pounded on the treatment room door until the surprised Therac-25 operator opened it. Dahl was immediately examined by a physician, who observed reddening of the skin but suspected only an electric shock. Dahl was discharged and told to return if he suffered any further complications.
The hospital physicist was called in to examine the Therac-25, but no problems were found. The Therac-25 was shut down for testing the next day, and two AECL engineers, one from Texas and one from the home office in Canada, spent a day at the ETCC running tests on the machine but could not reproduce a Malfunction 54. The home office engineer explained that the Therac-25 was unable to overdose a patient and also said that AECL had no knowledge of any overexposure accidents by Therac-25 machines. No electrical problems were found with the ETCC's Therac machine, and it was put back into use on April 7, 1986.
Isaac Dahl's condition worsened as he lost the use of his left arm and had constant pain and periodic nausea and vomiting spells. He was later hospitalized for several major radiation-induced symptoms (including vocal cord paralysis, paralysis of his left arm and both legs, and a lesion on his left lung). Dahl died in August of 1986 due to complications from the radiation overdose.
Victim #4: Daniel McCarthy
April 11, 1986. Technicians could find nothing wrong with the Therac-25 unit at the East Texas Cancer Center (ETCC), after the "Malfunction 54" incident that had injured Isaac Dahl. The machine was reinstated.Four days later, Daniel McCarthy was being treated for skin cancer on the side of his face. The same Therac operator who had treated Isaac Dahl was treating McCarthy. As the operator prepared to administer the Therac treatment from the control room, she used the "cursor up" key to correct an error in the treatment settings. She then began treatment using the "B" key.
The Therac-25 shut down within a few seconds, making a noise audible through the newly repaired intercom. The Therac monitor read "Malfunction 54." The operator rushed into the treatment room and found McCarthy moaning for help. He said that his face was on fire. The hospital physicist was called. McCarthy said that something had hit the side of his face, and that he had seen a flash of light and heard a sizzling sound.
After this second accident at the hospital, the ETCC physicist took the Therac-25 out of service and called AECL. He worked with the Therac operator who had been administering treatment to both Dahl and McCarthy when the accidents occurred. The physicist and the operator were eventually able to reproduce a Malfunction 54. They found that the malfunction occurred only if the Therac-25 operator rapidly corrected a mistake.
The ETCC physicist notified AECL of this discovery and AECL was eventually able to reproduce the error. AECL advised Therac-25 users to physically remove the up-arrow key as a short-term solution. AECL also filed a report with the United States FDA as required by law, and began work on fixing the software bug.
The FDA worked in conjunction with AECL to identify the software problem and correct it. The FDA also requested that AECL change the machine in several other ways to clarify the meaning of malfunctions error messages and to shut down treatment after any single large radiation pulse or interrupted treatment so that multiple overdoses were less likely.
Over the next three weeks Daniel McCarthy became very disoriented and then fell into a coma. He had a fever as high as 104 degrees and had suffered neurological damage. He died on May 1, 1986.
Victim #6: Anders Engman
January, 1987, Anders Engman was at the Yakima Valley Memorial Hospital on January 17, 1987 to receive three sets of radiation treatment from the Therac-25.The first two treatments went as planned. Engman received 7 rads (radiation absorbed dose), 4 rads followed by 3 rads of radiation to take pictures of internal structure. The Therac-25 operator then entered the room and used the Therac-25's hand control to verify proper beam alignment on Engman's body. Engman's final dose of the day was to be a moderate 79-rad photon treatment.
The operator pressed a button to command the Therac to move its turntable to the proper position for treatment. Outside the treatment room, the Therac-25's control console read "beam ready," and the operator pressed the "B" key to turn the beam on. The beam activated, but the Therac-25 shut down after about 5 seconds. The console indicated that no dose had been given, so the operator pressed "P" to proceed with the treatment.
The Therac-25 shut down again, listing "flatness" as the reason for treatment pause. Engman said something over the intercom, but the operator couldn't understand him. The operator went into the treatment room to speak with Engman. Engman told the operator that he had felt a "burning sensation" in the chest. The operator's console displayed only the total dose of the two earlier treatments (7 rads).
Later that day, Engman developed a skin burn over the treatment area. Four days later the burn was striped in a manner similar to that of Janis Tilman's burn after she had been treated at Yakima the year before.
AECL investigated the accident. All users were again told to visually confirm turntable setting before proceeding with any treatment. Given the information, it was suspected that the electron beam had come on when the turntable was in the field light position. AECL could not reproduce the error.
Later that week, AECL sent an engineer to Yakima to investigate. The hospital physicist had also been running tests. They eventually discovered a software flaw and fixed it. AECL engineers estimated that Engman received between 8,000 and 10,000 rads instead of the prescribed 86.
Anders Engman died in April 1987. He had been suffering from a terminal form of cancer before the Therac accident, but it was determined that his death was primarily caused by complications related to the radiation overdose, not the cancer.